hi @Mark_Phillips welcome to the forum 
likely you are looking for Nginx as “Reverse proxy”. basics see in 101: reverse proxy. You will find tons of topics related to reverseproxy and nginx
As it seems you are beginner I would recommend you take a look at GitHub - nextcloud/all-in-one: 📦 The official Nextcloud installation method. Provides easy deployment and maintenance with most features included in this one Nextcloud instance. - simple trunkey solution with build-in TLS certificates.